1. Field of Invention
The invention concerns an apparatus for improved wet oxidation of COD-containing effluents from industry and other activities containing chemical compounds which can be oxidized, especially oxidation of dissolved hydrocarbons, and is based on production experiences from the two above-mentioned inventions.
The apparatus is however also applicable for wet oxidation of other, COD-containing liquids in other connections, hereunder also biological, aerobic COD-reduction with activated sludge.
The apparatus is also applicable for anaerobic, biological COD-reduction with activated sludge, but then with shut off supply of oxygen and/or air.
2. Description of the Prior Art
Wet oxidation of oxidable compounds in aqueous solutions is well known as chemical reaction.
By 1960, descriptions of fullscale, technical plants for process industry and municipal installations were published. During the last 20-30 years more plants have been built, especially in USA and Europe.
The experience however from these plants is that they are expensive both in buying and operation. The wet oxidation process has therefore not found a widespread application appropriate to the demand, especially after the escalated environmental demands of the last few years.
This fact is based on the following 4 conditions:
1. The effluents to be oxidized are frequently very diluted, having a low COD-content, substantially below 20 g COD/1.
A very diluted effluent requires large heat quantities for heating to the required reaction temperature, in most cases exceeding 150.degree. C. A prerequisite for an economical wet oxidation is that the liquid preferably contains from 20 to 50 g COD/1 liquid.
2. Wet oxidation of industrial and municipal effluents requires frequently large quantities of oxygen, a fact which implies relatively high operational costs, regardless if the oxygen is added in a pure form or in a form of compressed air.
In order to become economically acceptable, the process must be able to maintain itself by its own heat generation, and preferably be able to deliver excess heat for other applications.
3. When using compressed air, the oxygen constitutes only ca 20% of the added air.
Today's wet oxidation plants operate with long reaction times, usually between 30 and 60 minutes. Such long retention time for a liquid-air-mixture in the system requires large volume within a big and closed reaction system.
Such a system becomes especially expensive due to the high pressures applied in order to "get into place" all the oxygen or the air, which quantity-wise must be adjusted to the COD-content to be oxidized.
The traditional plants of today frequently apply pressures in the order of 200 and 300 bars, a fact which besides making the plants expensive due to the pressure dimensioning also implies operational problems of technical and mechanical kind.
4. Today's plants for so-called biological COD-reduction with aerobic or anaerobic enzymatic processes operate very slowly with long reaction times which require voluminous and expensive plants.